m a x A I 4 3 0 & m a x A I 4 3 0 v T E C H N I C A L M A N U A L
50
Rev A 8/24/2020
4.4 Data-Bus Termination
The SAE specification for the J1939 data-bus requires data-bus termination. The J1708/1587 data-bus
does not require termination. Termination is required to attenuate any electrical noise developed by the
high-speed data transfer. If the termination resistors are not present, loss of data-bus communication may
occur.
Termination simply means installing two 120
½-Watt resistors between positive bus wire and negative
bus wire at each end of the data-bus backbone. If the installation involves connecting to an existing data-
bus, termination should already exist and no additional termination is required. The SAE specification
forbids the use of internal data-bus termination, because the data-bus must remain intact if any device is
removed. If a specific module terminated the data-bus and that module was removed, then the data-bus
could cease to operate.
The data-bus shield must also be terminated properly. Using a wire, the shield should be connected to
ground as close as possible to the battery ground. This termination connection to ground can be
anywhere along the data-bus, but it must only be connected at
ONE
point. The shield should also
connect to the shield pin of all data-bus devices (
NOT GROUND
).
Per the SAE J1939 specification, bus shielding is optional and may not be found on all systems.